1. Field of the Invention
The present invention relates to a power supply for a load control device, specifically, a multi-stage power supply for an electronic dimming ballast or light-emitting diode driver, where the power supply is able to operate in a low-power mode in which the power supply has a decreased power consumption.
2. Description of the Related Art
Typical load control devices are operable to control the amount of power delivered to an electrical load, such as a lighting load or a motor load, from an alternating-current (AC) power source. One example of a typical load control device is a standard dimmer switch, which comprises a bidirectional semiconductor switch, such as a triac, coupled in series between the power source and the load. The semiconductor switch is controlled to be conductive and non-conductive for portions of a half-cycle of the AC power source to thus control the amount of power delivered to the load. A “smart” dimmer switch comprises a microprocessor (or similar controller) for controlling the semiconductor switch and a power supply for powering the microprocessor. In addition, the dimmer switch may comprise, for example, a memory, a communication circuit, and a plurality of light-emitting diodes (LEDs) that are all powered by the power supply.
Another example of a typical load control device is an electronic dimming ballast, which is operable to control the intensity of a gas discharge lamp, such as a fluorescent lamp. Electronic dimming ballasts typically comprise an inverter circuit having one or more semiconductor switches, such as field-effect transistors (FETs) that are controllably rendered conductive to control the intensity of the lamp. The semiconductor switches of the inverter circuit are often controlled by integrated circuit or a microprocessor. Thus, a typical electronic dimming ballast also comprises a power supply for powering the integrated circuit or microprocessor.
By decreasing the amount of power delivered to an electrical load, a load control device is operable to reduce the amount of power consumed by the load and thus save energy. However, the internal circuitry of the load control device (e.g., the microprocessor and other low-voltage circuitry) also consumes power, and may even consume energy when the electrical load is off (i.e., the load control device operates as a “vampire” load). Thus, it is desirable to reduce the amount of power consumed by a load control device, and particularly, the amount of standby power consumed by the load control device when the electrical load is not powered.